This invention relates to a thermal cut-off fuse for breaking electric continuity between two lead wires at the time that the ambience of the fuse reaches a prescribed temperature, and more particularly to improvements in and relating to a thermal cut-off fuse using a temperature-sensitive member which melts at the prescribed temperature.
Many kinds of thermal cut-off fuses have heretofore been disclosed to the art. (U.S. Pat. Nos. 4068204, 4075595, 4075596, 4084147, 4127839, for example) As far as securing electric continuity between two lead wires under the normal working conditions below the prescribed temperature is concerned, those thermal cut-off fuses which use solid contacts are relatively safe and inexpensive. However, such conventional thermal cut-off fuses have required some kind of mechanical actuating means for dependably moving and isolating a contact means from at least one of the leads at the time the temperature-sensitive member reaches the prescribed temperature and melts. Generally, these actuating means incorporate at least spring means. The incorporation of such actuating means, therefore, has notably added to complication of the assemblage of smaller thermal cut-off fuses (those which are 3 to 5 mm in diameter and 7 to 10 mm in length, for example). Further from the operational point of view, variation in the charge imparted to the spring means has resulted, not infrequently, in mechanical failures. Moreover, the incorporation of these actuating means has the disadvantage that the total number of component parts involved are inevitably increased.
In the conventional thermal cut-off fuses using a temperature-sensitive member, the member melts upon reaching the prescribed temperature and the molten material flows into the gaps in the thermal cut-off fuse housings and, owing to its viscosity, locally retards the movements of the mechanical components so much as to impede normal functions of the fuses. As a result, even when the member melts at the prescribed temperature, the separation of the contacts of the fuse may occur only with some time lag. In the extreme cases, there is the possibility that the component parts of such fuses may fail to function normally to effect separation of the contacts as required.
Having issued from the realization of the state of affairs described above, the present invention has as one of its objects the provision of a thermal cut-off fuse which has a temperature-sensitive member and directly functions to effect separation of its contacts without use of a spring operated actuating means and which, therefore, functions safely over many years of service.
Another object of this invention is to provide a highly reliable thermal cut-off fuse which has a simple construction and uses a small number of component parts.